Dodgey batteries in tranmitter and receiver, powerlines, tractors and lorries with unsuppressed electrical systems, one street lamp next to our old house with a dodgey tranformer or whatever gubbins they have at the bottom, Using my exposure light at the same time (took a while to figure out that one), and a two year old fiddling with the reciever have all affected my hrm.

Hey mine does that, Went out for and early moring training when visiting my parents, whne i got back my Mum asked me how was the ride, so told her about the weird heart rate readings, she replied with 'oh that'll be your heart murmour' WTF! Apparently I've had it since I was a kid and now she tells me when im 31!

Never had problems with my basic polar (F5?) HRM. But it does give a different reading to the machines at the gym, as much as 5bpm, presumably the belt just sends out a beep every heartbeat and thyer using a different algorythm to calculate the average time between beeps?